The present invention relates to current mirror circuits that provide an output current which is ratioed to an input current and, more particularly, to a circuit for compensating base current errors in bipolar integrated current mirror circuits.
There are many applications for current mirror circuits in analog integrated circuits. Most commonly, a current mirror circuit or differential-to-single ended converter circuit is utilized in comparators and operational amplifiers. For example, most comparators and operational amplifiers include a differential input stage at which the outputs thereof there is generated a pair of differentially related currents. These related currents are converted to a single output current. In another application, a single input current may be supplied in which a single output current is desired that is ratioed to the input current. Most commonly, the current mirror or converter circuit is comprised of a diode-connected transistor coupled across the base-emitter of an output transistor wherein the former biases the latter. More particularly, an input current is supplied to the diode-connected transistor, which has its collector and base interconnected to the base of the output transistor while the emitters of the two devices are interconnected. By emitter area ratioing of the two transistors, an output current is developed at the collector of the output transistor that can be less than, greater than or equal to the input current. A problem with known current mirrors of the type described is related to base current error. Since a portion of the input current is utilized to provide the base current drive to the two transistors, the output current generally is not perfectly matched to the input current. There have been solutions proposed in the prior art to compensate for base current errors these solutions also present other problems. One known improved current mirror requires a fixed bias current and is not useful for use in operational amplifiers in which the bias current may be varied upon the operation of the amplifier. Another proposed solution requires at least two base-emitter voltage drops to be used which prevents the operational amplifier from providing maximum output voltage swing (using a single power supply).
Hence, a need arises for a precision current mirror circuit for compensating base current errors that has none of the disadvantages of the prior art.